A large number of surveillance, eavesdropping, goniometry systems or, more generally, systems detecting and processing electromagnetic signals are used to process waves originating directly from transmitters on the ground. Sometimes, however, ionospheric waves interfere with their operation, whereby said waves may be mistaken for signals emitted by a transmitter on the ground.
Ionospheric waves can propagate over a long distance, in the order of several thousands of kilometers. Furthermore, when the radio waves produced by a transmitter penetrate the atmosphere as far as the ionosphere, they are continually subjected to refraction phenomena which cause part of the energy of said waves to return towards the earth. A portion of this energy can then be picked up by a receiver, even if the latter is very distant from the initial transmitter. If, for example, this receiver is used by a goniometer in order to determine the direction of signals originating from nearby transmitters, the received ionospheric waves may adversely affect the obtained angle measurements through interference. Furthermore, independently of any interference phenomenon, an initial vertical polarization wave is transformed following ionospheric propagation into any given polarization wave, and is therefore likely to cause the failure of any radiogoniometry device designed to operate in an optimum manner in the face of zero elevation and vertical polarization incident waves. This occurs, for example, in the case of “Watson-Watt” goniometry.
It is therefore desirable to distinguish ionospheric waves from sea and/or ground waves. Furthermore, in the case of goniometry measurements, it is desirable to evaluate the validity of said measurements according to the type of wave received by the goniometer antenna (ionospheric wave or ground/sea wave).